Air surveillance systems (also referred to herein as radar systems) are designed to produce an accurate display of information related to flying objects in a selected field of operation (i.e., an airspace).
FIG. 1 is a block diagram of a conventional radar system 100, which comprises hardware 105 (including a transmitter 110, a receiver 120 and an antenna 125), a processor 130, a target tracker 140 and a display 150. Transmitter 110 emits radio waves, detector 120 receives the radio waves after they are reflected from an object O and converts the waves into an electric signal, and processor 130 is programmed to analyze the received signals. Processor 130 identifies signal characteristics (e.g., signal strength and Doppler values) and identifies the characteristics with a location in space (i.e., a cell) and a time of detection.
The airspace of a radar system may be may be divided into 100,000 or more cells, each cell being identified using a radial value and an azimuthal value. A radar system is designed to determine, with the desired accuracy, for each cell, whether there is relevant information (i.e., a flying object) to be displayed. A signal that has been identified as coming from a flying object is referred to herein as a “hit.”
To avoid displaying non-relevant objects (e.g., geographical features, weather, and noise), processor 130 of a radar systems is commonly provided with clutter suppression features. To avoid displaying noise, conventional radar systems, such as system 100, include a constant false alarm rate (CFAR) system. A CFAR system is conventionally implemented as either a cell averaging threshold (i.e., a spatial average), or a time averaging threshold via a finite impulse response filter, to eliminate signal values associated with noise, which is relatively low compared to reflected signals originating from transmitter 110. Accordingly, the signal strength threshold of the CFAR system is relatively low. To suppress signal returns from stationary objects (e.g., mountains, buildings and other ground clutter), radar systems may be provided with a coherent moving target indicator (MTI) or a coherent Doppler processor; and a clutter map may be present to address slowly changing artifacts such as those that might arise due to solar heating or weather phenomena. Particularly troublesome for conventional radar systems are non-flying objects that move at relatively high speeds and present varying radar reflection characteristics (e.g., varying signal strength and/or Doppler speeds), but which are not relevant for display, such as windmills, and wind-swept tree leaves and tall grass. Such objects are referred to herein as moving clutter.
To achieve an accurate representation of relevant objects requires, both, avoidance of display of non-relevant information and avoidance of omission of relevant information. Typical design goals require that a radar system detect and display an object on 90% of chances (i.e., in 9 out of ten scans), and avoid displaying information in the event that nothing is present (i.e., avoid false alarms) 99.999% of the time averaged over all cells in the airspace in aggregate.
Various enhancements to clutter suppression systems have been added to address the issue of windmills. For example, such systems may simply filter based on the Doppler velocity or process outputs using signals from two radar systems or recognize and eliminate signals corresponding to a known reflection signature or alter the operation of the windmill to facilitate detection and filtering.
Such enhanced systems have limitations in ability to accurately display data, as well as limitations in their ease of implementation.